In mobile/cellular network performance analysis, the rate of missed calls for a user is a Key Performance Indicator (KPI), and there are a number of situations where a missed call may occur, including where:                (a) the called user's mobile terminal was switched off: i.e. turned off by the user or by the terminal itself in the event of a depleted battery, both of which result in the terminal sending a network detach message before shutting down; this case also includes the de-activation of mobile network access without shutting down the terminal (e.g., flight-mode).        (b) the called user was busy: i.e. on another call or not answering the ringing terminal; and        (c) the called user was not reachable/non-contactable by paging procedure indicating out of coverage or other conditions with the same effect: i.e. terminal attached to the network, but non-responsive to the paging message sent by the network on the occurrence of the incoming call.        
From a customer viewpoint, the first two missed call situations, (a) and (b) listed above, are generally acceptable, since they are within the user's control. The third point (c), however, is out of the user's control, and therefore likely to cause dissatisfaction and be viewed as a service inadequacy.
The problem of the terminal entering a non-reachable state may be due to a number of issues, such as:                i) network coverage extent or continuity problems (i.e. the user of the mobile terminal may have moved and entered a region with no signal coverage or with only a weak signal coverage);        ii) user's terminal associated problems (e.g. terminal antenna sensitivity, performance of the terminal receiver, etc.);        iii) the network protocols and standards associated problems (e.g. 3GPP operational standards);        iv) rarely occurring conditions e.g. local interference in paging channels; or        iv) any combination of the above.        
The nature of the out-of-coverage problem varies depending on the specific situation e.g. a user moving in remote rural areas where there is no coverage may experience long periods of out-of-coverage status while other cases, like entering the elevator or an underground parking, may lead to out-of-coverage situations of limited duration. It should be noted that according to the GSM and UMTS standards as soon as the terminal returns back to coverage then a location registration procedure is initiated notifying the network that the terminal is attached and reachable.
A common problem for mobile terminals with a choice of coverage occurs when the terminals are in idle mode. In idle mode, mobile terminals are configured to select the best available service cell (cell selection and re-selection procedure). Therefore, when in a region where good coverage of multiple cells is available (e.g. from both 2G cells and 3G cells), a 2G/3G terminal may be likely to regularly switch between the 2G and 3G cells depending on the 3G signal quality. This switching, although generally transparent to the user, does result in the terminal being out of coverage and not reachable for a very short period of time during this cell reselection process (it may reach the order of a few seconds). In cases of problematic 3G signal strength levels the terminal can regularly be switching between 2G and 3G cells, leading to a phenomenon where this short out of coverage time period is effectively accumulated and multiplied. The actual period of time that the terminal is out of coverage typically varies depending on the operation software on the terminal, and accordingly the terminal type. During this period, any incoming calls would not be able to connect with the terminal, and a missed call due to coverage problems would result. With the introduction of 4G systems (e.g. Long Term Evolution or LTE technology) where the terminals will switch between 2G, 3G and 4G cells this type of problem may become even more intense and annoying for the users.
Out of coverage situations are difficult to analyse based on network measurements, particularly in terms of their duration, because when terminals are out of coverage they have no interaction with the network.
There is therefore a need to be able to better recognise and analyse problems arising from missed calls, particularly those due to out of coverage events.
There is also a need to be able to better provide customer centric management for telecommunication network providers.
Customer centric management is an area of network data analysis which considers the ability to provide KPIs on a per MSISDN basis (i.e. a per user basis) in order to provide a characterisation of individual customer experience. This analysis is very useful, particularly for addressing customer complaints (such as due to a large number of missed calls outside their control), and also tracking the Service Level Agreements (SLAs) offered to corporate customers.
To date customer centric management systems typically rely on two main techniques, namely Call Detail Record (CDR) analysis (for off-line analysis of customer experience) and passive probing systems (for near real time analysis of customer experience). These techniques allow for the analysis of a wide variety of parameters, however they do not actually capture the events associated with incoming calls while the called terminal is out of coverage. More specifically, for CDR analysis, out of coverage analysis is not possible because such information is not included in the CDR. For passive probing systems out of coverage analysis would only be possible on a complex series of settings to the Visitor Location Register (VLR) nodes. These settings would not only be complex to implement, but would only be able to be implemented on a limited scale since such a configuration would result in an additional VLR processing load. This is undesirable, and so there is also a need for an improved analysis technique.